A variety of materials and construction methods have been used in connection with turbine airfoils. For example, laminated airfoil concepts are known that use monolithic ceramic materials. Reasons for using such constructions include the reduction of impact stresses, reduction of thermally induced stresses from differential cooldown rates (e.g., thin trailing edge sections versus thicker sections), and accommodation of attachment to metals. However, precise and costly machining of individual laminates preclude the viability of these concepts.
Another type of material used in connection with turbine airfoils is ceramic matrix composites (CMC). CMC includes a ceramic matrix reinforced with ceramic fibers. In one CMC airfoil construction, fabric layers are wrapped over each other so that the fibers are primarily aligned substantially parallel to the surface of the component. For a 0/90 degree fabric lay-up, the fibers in the vane would substantially be oriented parallel to the gas path around the vane and along the vane radially to the machine. Furthermore, the reinforcing fibers are continuous and form an integral shell.
CMC airfoil designs can provide advantages over the monolithic airfoils described above. For example, the higher strength and toughness of CMCs can resolve the impact and thermal stress issues associated with monolithic ceramics, and their superior strain tolerance makes them more amenable to attachment to metal structures.
While providing some advantages over monolithic ceramics, the use of CMC materials in airfoil design introduce a new set of challenges. For example, CMC materials suffer from their low interlaminar tensile and shear strengths, which present special challenges in situations where an internally cooled component, such as a turbine vane, experiences large through thickness thermal gradients and the resultant high thermal stresses. In the above-described CMC airfoil construction, high thermal gradients cause high interlaminar tension (i.e. high stresses) in the weakest direction of the CMC material, resulting in delamination of the CMC.
Prior attempts to mitigate these stresses include three dimensional fiber reinforcement and exotic cooling methods. However, these approaches carry numerous development and manufacturing disadvantages and performance penalties.
Further, prior CMC airfoil constructions pose various manufacturing challenges. For instance, current oxide CMCs exhibit anisotropic shrinkage during curing, resulting in interlaminar stress buildup for constrained geometry shapes. Further complicating matters is that non-destructive evaluation methods to discover interlaminar defects are difficult on large, complex shapes such as gas turbine vanes. In addition, dimensional control is unproven for complex shapes and may be difficult to achieve in close-toleranced parts such as airfoils. Further, achievement of target material properties in large and/or complex shapes has proved to be difficult. There are also scale-ability limitations as current processes are labor-intensive, requiring very skilled technicians to carefully hand lay-up each reinforcing layer. Conventional lay-up techniques provide low pressure containment capability for trailing edge regions. In one example, the reinforcing fabric wrapped around the pressure and suction sides of the vane meet at the trailing edge where they become tangent to each other and are bonded together in the same manner as each layer is bonded to the adjacent layer. Consequently, the trailing edge is only weakly held together and is vulnerable to the pressure of the cooling air in the trailing edge exit holes.